With continuing progress in miniaturization in electronic components and devices comes new challenges and opportunities in assembly and packaging of electronic modules and devices. For many applications integrated circuit (IC) dies are surface-mounted to the circuit board, flex circuit or other substrate (referred to collectively as "substrate(s)" herein) where they will be used, instead of first packaging the dies in chip carriers. This permits more dense packaging where physical size and short signal paths are a concern.
The industry standard for electrical connections to an IC die is a number of connection pads which can be wire-bonded, on a surface (often referred to as the "top" surface, regardless of the orientation in which the chip will be mounted) of the chip. However, some common types of mounting of ICs require special preparations for the bond sites prior to mounting. One such method, referred to as "flip chip" mounting, involves surface mounting the ICs inverted so that their tops are on the substrate with their bond pads for signal and power connections facing or contacting the substrate. A common technique is to prepare the bond pads on the IC with solder bumps or gold bumps, which then contact corresponding signal or power traces or pads on the substrate. Soldering the bumps to the substrate provides electrical and mechanical connection for the IC.
More recently, various types of conductive polymers have been used instead of solder or gold for making these connections. In such systems the polymer is applied to the IC bond pads, the substrate traces, or both, and serves to bond and make electrical connections between the IC and substrate when the IC is placed on the substrate. Some polymer methods do not require special treatment of the bond pads of the IC, which is an advantage.
It has been generally recognized that for mechanical security and resistance to mechanical and thermal stress, and IC mounted to a substrate by one of the above techniques needs to be underfilled to fill the void which would otherwise be left between the surface of the IC and the substrate supported by the bonding of the bond pads. Various methods have been proposed to use a combination of conductive and non-conductive polymers. The conductive polymer is used for the electrical connections to the IC, while the non-conductive polymer is used as the attachment and underfill for the IC. Such techniques involve the application by printing steps of patterns of conductive and non-conductive polymers (or monomers which are subsequently polymerized) on the IC and/or the substrate, according to the specific locations of the ICs and the electrical contact traces. However, it can be difficult in such systems to achieve the necessary precision in registration. Other methods in have been developed for forming the underfill from an epoxy or some other non-conductive polymer. One technique is to apply the underfill in liquid form to the substrate in a dispensing step after mounting the ICs. However, mounting the IC first then underfilling using the capillary action of a non-conductive polymer is a relatively slow process. Another technique is to provide a hole in the substrate under the IC through which liquid epoxy is dispensed to form the underfill. Others have proposed using a vacuum technique for applying the underfill.
Although the above-described techniques provide useful alternatives for mounting of ICs, the present invention provides further improvements in terms of simplicity and efficiency of process, and integrity of the mechanical and electrical connections of components to the substrate.